1. Field of the Invention
The present invention relates to a control of a steplessly variable transmission for motor vehicles, and more particularly to a control system for a steplessly variable transmission wherein the speed ratio of the transmission is controlled in accordance with vehicle operating conditions. More specifically, the present invention pertains to a transmission speed ratio control for making it possible to develop an engine brake function.
2. Description of Prior Art
In motor vehicles having steplessly variable transmissions, it has been proposed to provide measures for making it possible to develop an engine brake function. For example, Japanese patent publication No. 45-23768 discloses a speed ratio control system for a steplessly variable transmission which includes a hydraulic actuator for controlling the speed ratio of the transmission and a control valve for controlling the hydraulic pressure to the actuator in accordance with the engine speed and the position of the engine throttle valve so that the speed ratio can be adjusted in accordance with the engine operating condition. In order that an effective engine brake function can be developed in an operation wherein the engine throttle valve is closed, the Japanse patent publication proposes a mechanism in which the control valve is shifted to one extreme position when the engine throttle valve is closed so that the actuator is moved to the position wherein it maintains the highest speed ratio of the transmission.
The system as proposed by the Japanese patent publication thus provides a predetermined, that is, the highest speed ratio of the transmission when the engine throttle valve is closed so that a maximum rate of engine brake function is always obtained. The proposed system, however, cannot satisfactorily comply with actual demands in vehicle operations. In fact, demands for engine brake functions depend on vehicle speeds. For example, a greater engine brake function is desired under a high vehicle speed than under a low vehicle speed so that there is a demand to change the rate of engine brake in accordance with the vehicle speed.